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Flow manipulation is a critical expected capacity for integrated microsystems. One way to realize low cost devices is to take advantage of capillary forces for fluid movement. In such systems, flow manipulation should be achieved with easily operated, effortlessly integrated valves. One advantageous method to operate sensors and actuators in microsystems is electrochemistry. Here, the design, fabrication and implementation of low voltage electrochemically actuated passive stop-go microvalves for on-off fluidic manipulations in microfluidic systems are reported. Two closely spaced electrodes (one of which has superhydrophobic surface) were fabricated by screen printing followed by surface structuring in a microchannel. The fabrication of the superhydrophobic surface (water contact angle (CA) 152^o) was performed by selective and controlled solvent-etching of a naturally hydrophobic (CA=105^o) screen printed carbon surface. The process increased the roughness and porosity of the surface that caused the superhydrophobic effect. The superhydrophobic surface of the carbon electrode, in addition to functioning as passive stop valve (PSV), facilitates the flow actuation using low applied voltage avoiding observable electrochemical reactions in aqueous solutions. When a low voltage (~1V) was applied at the carbon electrode against a silver electrode, the flow of such solutions (e.g. 0.01M phosphate buffer saline solution) that was stopped at the PSV, resumed, crossing the 1mm pitch of hydrophobic barrier of the PSV in 1s and reestablishing capillary flow downstream. The low cost and flexibility of fabrication, facile integration and miniaturization, and reproducible performance of such on/off valves make this configuration promising for the development of low cost microfluidic devices for point-of-care diagnostics, food analysis, and environmental monitoring.

10.1016/j.electacta.2012.12.110 Screen printed carbon electrodes provide attractive opportunity for the development of miniaturized low cost electrochemical sensors. However, the electrodes display very low level of electrochemical performance due to the nonelectroactive components in the ink formulation and relatively low graphitic carbon content imposed by the constraints of screen-printing. In this work, selective etching action of a judiciously selected organic solvent was envisaged to improve the electrochemical characteristics of the electrodes. The procedure involves soaking the electrode strips in N,N-dimethylformamide (DMF) for just minutes of etching followed by drying. Five minutes DMF treated electrode, for instance, showed a 100-fold increase in the heterogeneous charge transfer kinetics compared to the untreated electrodes. The effective area of the solvent treated electrode was 57-fold of its apparent geometric area. The solvent treated electrodes also showed a dramatically decreased charge transfer resistance from ca. 18,000 O to 180 O. The results of surface examination using scanning probe microscopy and electrochemical impedance spectroscopy revealed enhancement in the porosity of the solvent treated electrodes. The procedure is versatile and applicable to other screen-printed electrodes such as gold, silver or silver/silver chloride to improve the performances characteristics including stability. The ability to activate several electrodes simultaneously and multiple re-usability of the solvent makes this approach cost effective and very promising to facilitate mass production of miniaturized and disposable electrochemical sensors.

In this study, we developed a potentiometric planar strip cell based on single-walled carbon nanotubes that aims to exploit the attributes of solid-contact ion-selective electrodes for decentralized measurements. That is, the ion-selective and reference electrodes have been simultaneously miniaturized onto a plastic planar substrate by screen-printing and drop-casting techniques, obtaining disposable strip cells with satisfactory performance characteristics (i.e., the sensitivity is 57.4 ± 1.3 mV/dec, the response time is ≤30 s within the linear range from log a(K+) = -5 to -2, and the limit of detection is -6.5), no need of maintenance during long dry storage, quick signal stabilization, and light insensitivity in short-term measurements. We also show how the new potentiometric strip cell makes it possible to perform decentralized and rapid determinations of ions in real samples, such as saliva or beverages.

Development of a fully automated electrochemiluminescence (ECL) DNA assay for multiplex detection of six biowarfare agents is described. Aminated-DNA capture probes were covalently immobilised on activated-carbon electrodes and subsequently hybridised to target strands. Detection was achieved via a sandwich-type assay after Ru(bpy)3 2+-labelled reporter probes were hybridised to the formed probe–target complexes. The assay was performed in an automated microsystem in a custom-designed ECL detection box with integrated fluidics, electronics, and movable photomultiplier detector. The obtained limits of detection were 0.6–1.2 nmol L−1 for six targets ranging from 50 to 122 base pairs in size, with linear range 1–15 nmol L−1. Non-specific adsorption and cross-reactivity were very low. Detection of six targets on a single chip was achieved with subnanomolar detection limits.

Controlled Zn-mediated grafting of a thin layer of a diazonium salt was used to functionalise a carbon electrode with ruthenium(II)-tris-bipyridine (Ru)-labelled DNA for use as a capture probe in an electrochemiluminescent genosensor. A secondary reporter probe was labelled with a ferrocene (Fc) molecule, and in the presence of the single-stranded DNA target a genocomplex formed, where the Fc-label effectively quenched the electrochemiluminescence of the signal emitted from the Ru-label. The spacing of the labels for maximum sensitivity and minimum detection limit was optimised, and the signal reproducibility and stability of the method was established.

In this technical note, we report a new all-solid-state planar reference electrode based on single-walled carbon nanotubes and photocured poly(n-butylacrylate) (poly(nBA)) membrane containing the Ag/AgCl/Cl(-) ion system. Single-walled carbon nanotubes functionalized with octadecylamide (SWCNT-ODA) and deposited by drop-casting onto a disposable screen-printed electrode are an excellent all-solid-state transducer. The novel potentiometric planar reference electrode shows low potential variability (calibration slopes inferior to 2 mV/dec) for a wide range of chemical species (i.e., ions, small molecules, proteins) in a wide calibration range, redox pairs, changes in pH, and changes in ambient light. Potentiometric medium-term signal stability (-0.9 ± 0.2 mV/h) and electrochemical impedance characterization confirm the correct solid contact between the SWCNT-ODA layer and photocured poly(nBA) membrane. Overall, the materials used and the simple fabrication by screen-printing and drop-casting enable a high throughput and highly parallel and cost-effective mass manufacture of the new disposable reference electrode. Moreover, the reference electrode has a long shelf life, a characteristic that can be of special interest in decentralized and multiplexing potentiometric analysis.